Of 957 patients with type 1 diabetes without known Addison's disease 1.6% (15) were positive for 21-hydroxylase autoantibodies. Among DQ8/DQ2 heterozygous patients, the percentage expressing 21-hydroxylase autoantibodies was 5% (10 of 208) vs. less than 0.5% of patients with neither DQ8 nor DQ2. Three of the diabetic patients found to have 21-hydroxylase autoantibodies on screening were subsequently diagnosed with Addison's disease. Overall, the genotype DQ8/DQ2, consisting of DRB1*0404/DQ8 with DRB1*0301/DQ2, was present in 14 of 21 patients with Addison's disease (8 of 12 with diabetes and 6 of 9 without diabetes or anti-islet autoantibodies) vs. 0.7% of the general (109 of 15,547, P<10-8) and 11% of patients with DM without Addison's disease (62 of 578, P<10-8). Among patients with diabetes with DQ8, Addison's disease was strongly associated with the specific DRB1 subtype, DRB1*040 (8 of 9 patients from 8 families, in contrast to only 109 of 408 DQ8 DM patients with diabetes without Addison's disease having DRB1*040; P<.001). Among 21-hydroxylase autoantibody-positive DQ8 patients, 80% with DRB1*0404 (12 of 15) had Addison's disease, in contrast to 1 of 10 autoantibody-positive patients with DRB1*0401 or DRB1*0402 (P>.001). We conclude that patients with DRB1*0404 and 21-hydroxylase autoantibodies are at high risk for Addison's disease. Patients with DRB1*0401 and DRB1*0402 have more limited progression to Addison's disease despite the presence of 21-hydroxylase autoantibodies.